The present invention relates to a seal for a cylindrical surface which is used as a shaft seal devices.
One example of the seal of this kind has been heretofore known as shown in FIGS. 18 to 20, wherein a segment seal 1 is brought into sliding contact with a liner 14 on the side of a shaft 11.
More specifically, this segment seal 1 has a housing 2 which houses therein a seal element (seal ring kit 3) comprising the combination of a seal ring 4, a cover ring 5, a key 6, an extension spring (garter spring 7) and a compression spring (coil spring 8), which is covered with a spring retainer (spring adapter 9) and fixed by a snap ring 10. The seal ring 4 and the cover ring 5 are principally formed of a carbon material, and both the rings 4 and 5 are equally divided into more than two sections depending on the size of a shaft 11, which are combined with the divisional phase deviated in the circumferential direction. A divisional portion of the seal ring 4 is formed into a step joint 12 in order to prevent leakage in an axial direction. The seal element 3 is stopped in rotation through the key 6 by means of a rotation lock pin 13 disposed upright on the housing 2, and held on the shaft 11 by means of the extension spring 7 so as to be able to follow the radial movement of the shaft 11. The internal diameter surface of the seal ring 4 comes into sliding contact with the external diameter surface of a liner 14 slipped over the shaft 11 to constitute a sliding seal portion (a dynamic seal portion 15) which forms a primary seal portion. The seal element 3 is axially biased by the compression spring 8, and the perpendicular end thereof is pressed against the end wall of the housing 2 to constitute a static seal portion 16 which forms a secondary seal portion. The aforesaid internal diameter surface and perpendicular end which form both the seal portions 15 and 16, respectively, are formed with pressure balance grooves 17 and 18 to relieve loads resulting from fluid pressures. FIG. 21 shows one example of the pressure balance groove 17 formed in the internal diameter surface. The liner 14 in sliding contact with the internal diameter surface of the seal ring 4 is generally formed of hardened steel, and the external diameter surface is finished into a flat surface.
The segment seal 1 constructed as described above is used to seal various fluids. However, in the case where a gas G is sealed by this segment seal 1, a sliding seal portion 15 operates under a dry condition, and therefore the resulting from sliding cannot be sometimes removed efficiently. To cope with this situation, in the segment seal 1, the internal diameter surface of the seal ring 4 is formed in advance with the pressure balance groove 17 to positively introduce gas pressure therein to keep the seal surface pressure lower so as to prevent the heat result from sliding from exceeding a level above a limit value. However, when a differential pressure between both sides of the seal exceeds a level above the functional limit of the lowering the surface pressure caused by the pressure balance groove 17 or when the number of revolutions of the shaft increases, sliding heat in excess of the heat limit of the sliding material occurs. Under the condition as described in excess of the heat limit, liquids (cooling liquids) such as oil or water are sprayed by a jet against the shaft portion in the vicinity of the sliding seal portion 15 on the low pressure side to cool the sliding seal portion 15.
Further, in jet engines for aircraft, gas compressors or gas turbines, etc., high pressure gas such as compressed air, combustion gas and the like flow into bearing portions failing to properly lubricate the bearings. Therefore, it becomes necessary to provide a seal with a partition between a bearing chamber and a high pressure gas portion, and the segment seal 1 is used for that partitioning portion. FIG. 22 shows a construction of a bearing chamber 19 and peripheral portions thereof of a jet engine on which the segment seal 1 is mounted for the reason as described above. The segment seals 1 are mounted on both axial sides so as to isolate the bearing chamber 19. Oil for lubrication and cooling of a bearing 20 is supplied by jets 21 and 22 into the bearing chamber 19, the oil also having a function to cool the segment seal 1.
As described above, the segment seal 1 is incorporated into a variety of devices to seal gas, and the segment seal 1 is sometimes used under the situation that in the working atmosphere, the front receives high pressure gas and the back receives low pressure liquid, as described above. Under these circumstances, when the pressure difference between the gas and liquid is small, the liquid enters the sliding seal portion 15 and the liquid sometimes further leaks into the gas side.
The phenomenon wherein when the pressure difference is small, the low pressure liquid leaks into the high pressure gas side is explained as follows:
The case of the aforementioned jet engine is here taken as an example. Oil O in the form of mists for lubrication and cooling of the bearing 20 and cooling of the sliding seal portion 15 is present on the low pressure side of the segment seal 1, and the seal 1 is in an atmosphere wherein the oil O leaks, and the oil O also enters the seal element 3. When the pressure difference between the gas G and the oil O is large, the gas pressure overcomes the surface tension of the oil O and enters a clearance of the seal element 3 to prevent further entry of the oil O, whereas when the pressure difference becomes low, particularly, less than 0.3 kgf/cm.sup.2, the surface tension of the oil O becomes greater than the gas pressure so that the oil O enters even the interior of the seal element 3, which has been experimentally understood. Particularly, in the segment seal 1, the leakage of the gas G is concentrated on the divisional portion of the seal element 3 in terms of the divisional construction of the seal element 3, and therefore in the circumferential seal portion (sliding seal portion 15 and static seal portion 16) other than the divisional portion, leakage of the gas G is extremely small, and the oil O is liable to enter that portion. In addition, in the sliding seal portion 15 in sliding contact with the liner 14, oscillations resulting from the rotation of the shaft 11 occur, and therefore, a variation in surface pressure occurs in the sliding seal portion 15 in terms of the following operation with respect to the oscillations of the seal element 3, as a result of which the state of the oil film of the entered oil O tends to change. Furthermore, in the sliding seal portion 15, the seal ring 4 assumes a state wherein the seal ring 4 is levitated from the liner 14 (just like the state wherein a surfboard rides on the waves) due to the oil film formed on the portion 15 to further promote an entry of the oil O and a formation of an oil film. The oil film formed on the sliding portion 15 is scraped at the divisional portion of the seal ring 4 as the shaft 11 rotates and leaks into the gas G side.
In the prior art, it is considered that in order to effectively operate the segment seal 1 under the condition of low pressure difference as described above, two opposed seal elements 3 are used, as shown in FIG. 23, to feed high pressure gas (air or the like is suitable) into an intermediate chamber 23 to intentionally create the condition of a high pressure difference. However, in this case, a space to some extent is required and the construction is complicated, which is not an adequate measure. Alternatively, if this measure is used on a jet engine, the high pressure gas is bled from the compressor portion of the engine, resulting in a complex engine construction of the engine and the lowering in efficiency caused by bleeding.